1. Field of the Invention
The present invention relates to detecting means for ophthalmic instruments. More specifically, the present invention pertains to means for detecting images of marks projected to patients' eyes by ophthalmic instruments.
2. Description of Prior Art
Hithertofore, it has been proposed in the field of ophthalmic instruments to project an image of a mark to the retina of the patients' eye so that the focus condition of the photographing optical system of the instrument can be judged by the projected image of the mark. There has further been proposed to detect the projected mark image electrically by producing electric signals corresponding to beams of light from the retina of the patients' eye to which the mark image is projected and slicing the electric signals by a predetermined threshold level to detect the electric signals corresponding to the projected mark image. Such electric signals representing the mark image may be utilized for example in an automated focusing system. It should however be noted that the electric signals as produced by the light beams from the eye retina vary in levels and S/N ratios depending on the reflectivity of the retina, the intensity of the illuminating system and/or various other factors. Such variations can cause inaccuracy in the detection of the mark image and lead to a decrease in the reliability of the automated focusing.